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Conductivity data of the xAgI(1 - x)AgPO(3) system (0 ≤ x ≤ 0.5) were collected in the liquid and glassy states. The difference in the dependence of ionic conductivity on temperature below and above their glass transition temperatures (T(g)) is interpreted by a discontinuity in the charge carrier's mobility mechanisms. Charge carrier displacement occurs through an activated mechanism below T(g) and through a Vogel-Fulcher-Tammann-Hesse mechanism above this temperature. Fitting conductivity data with the proposed model allows one to determine separately the enthalpies of charge carrier formation and migration. For the five investigated compositions, the enthalpy of charge carrier formation is found to decrease, with x, from 0.86 to 0.2 eV, while the migration enthalpy remains constant at ≈0.14 eV. Based on these values, the charge carrier mobility and concentration in the glassy state can then be calculated. Mobility values at room temperature (≈10(-4) cm(2) V(-1) s(-1)) do not vary significantly with the AgI content and are in good agreement with those previously measured by the Hall-effect technique. The observed increase in ionic conductivity with x would thus only be due to an increase in the effective charge carrier concentration. Considering AgI as a weak electrolyte, the change in the effective charge carrier concentration is justified and is correlated to the partial free energy of silver iodide forming a regular solution with AgPO(3).

The mixed alkali effect, the deviation from expected linear property changes when alkali ions are mixed in a glass, remains a point of contention in the glass community. While several earlier models have been proposed to explain mixed alkali effects on ionic motion, models based on or containing discussion of structural aspects of mixed-alkali glasses remain rare by comparison. However, the transition-range viscosity depression effect is many orders in magnitude for mixed-alkali glasses, and the original observation of the effect (then known as the Thermometer Effect) concerned the highly anomalous temperature dependence of stress and structural relaxation time constants. With this in mind, a new structural model based on topological constraint theory is proposed herein which elucidates the origin of the mixed alkali effect as a consequence of network strain due to differing cation radii. Discussion of literature models and data alongside new molecular dynamics simulations and experimental data are presented in support of the model, with good agreement.

Calcium aluminosilicate (CAS) glass systems are industrially significant due to their favorable optical, mechanical, and thermal properties, as well as serving as a basis for many glasses in nuclear waste confinement and alkali-free display substrates. [1][2][3][4] The properties which make them industrially desirable are closely linked to their complex structure. In general, the glass system is comprised of network forming silica and alumina units, both ideally in tetrahedral arrangement with four bonded oxygens. Bonded oxygens which bridge to neighboring network formers are appropriately called bridging oxygens (BOs). An interesting feature of the CAS glass system occurs when varying the ratio, R, of [Al 2 O 3 ]/[CaO]. When R < 1 (i.e., percalcic regime), calcium cations act as charge compensators for the negatively charged (AlO 4/2 ) − tetrahedra. Excess calcium cations decrease network connectivity, resulting in non-bridging oxygens (NBOs). At R = 1, previous studies consider the network to be fully connected with no NBOs. 5 However, multiple studies have revealed that a finite concentration of NBOs exist at this ratio. 6,7 When R > 1 (i.e., peraluminous systems), the [AlO 4/2 ] − units are in excess, with insufficient calcium cations available for charge balancing. Two mechanisms have been proposed to compensate for the insufficient population of modifier cations: the formation of highly coordinated alumina (i.e., [5] Al and/or [6] Al), [8][9][10][11] or the formation of three-bonded oxygens (TBO), also known as triclusters. 12 While triclusters are present in crystalline polymorphs, 12,13 verification of triclusters in glassy systems has been limited spectroscopically, and hence molecular dynamics (MD) has been the primary

Ionic conductivity is a property of rapidly increasing interest. Various models attempting to explain ionic conductivity of glass systems have shown limited agreement with experimental results; however, none have been comprehensive. By using molecular dynamics simulations, the diffusion of ion species through a network can be directly observed, providing insights into the mechanisms and their relation to ionic conductivity models. In this report, a method of utilizing molecular dynamics simulations is proposed for the study of the ionic mobility of Na, Li, and K ions in binary silicate glasses. Values found for glasses with x = 0.1, x = 0.2, and x = 0.3 alkali content are between 10 −5 and 10 −4 cm 2 •s −1 •V −1 and did not change significantly with composition or temperature. This is in agreement with the interstitial pair and weak-electrolyte models used to explain ionic conductivity in glasses.